When faced with a strange problem like this, it helps to strip as much out as possible, to try to get to a small example that demonstrates the problem.

Here's the example I came up with:

#/usr/bin/perl -T

$insecure = $ARGV[0];

eval makeformat($insecure);
write;

sub makeformat {
    my($name) = @_;        # note: $name is tainted

    return "format = \n" .
           "\@<<<<\n" .
           '$' . $name . "\n" .
           ".\n"

    # the returned format string is tainted
}
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The implication is that you're constructing one of your format strings using a variable (or variables) that are still tainted.